1. Field of the Invention
The present invention generally relates to a pointing device, and in particular to a track ball based pointing device of which a biasing force is indirectly applied to the track ball by means of a sphere.
2. The Related Art
Pointing devices, such as a computer mouse, comprise a casing inside which a track ball is rotatably received. The track ball engages and frictionally drives a first axle (X-axis) and a second axle (Y-axis) to generate signals corresponding to movement of the mouse which in turn controls the movement of a cursor on a display screen. To ensure the frictional engagement between the track ball and the axles, biasing means is provided to force the track ball against the axles. Conventionally, the biasing force is applied indirectly to the track ball by means of a rotatable shaft. In other words, the rotatable shaft is biased against the track ball and the interaction between the shaft and the track ball in turn forces the track ball against the first and second axles.
When the track ball is rotated in a direction substantially inconsistent with the rotational direction of the shaft, the track ball is subject to a resistance caused by friction induced by relative movements of the track ball and the shaft in different directions. The movement of the cursor on the display screen becomes unstable and is not synchronous with the mouse, leading to incorrect operation of the computer.
Thus, it is desired to have a pointing device that does not have the deficiency of the conventional pointing devices.